1. Field of the Invention
This invention relates to a liquid friction coupling comprising a housing, which constitutes one coupling part and is filled with a viscous liquid, a shaft, which protrudes into said housing and constitutes the other coupling part, interdigitating disks, which are nonrotatably connected in alternation to the housing and to the shaft, respectively, and spacers disposed between said disks.
2. Description of the Prior Art
Liquid friction couplings of that kind are used, e.g., in vehicle drives. They have the advantage that the torque transmitted by the coupling will increase as the differential speed is increased.
Conventional liquid friction couplings have the disadvantage that at a given differential speed the torque being transmitted by the coupling will decrease in response to the temperature rise which is due to the heat generated by viscous friction. As a result, the torque will be too high at a low temperature, when the coupling begins to slip, and will be too low at a high temperature.
To oppose that undesirable torque change it is known, e.g., from Austrian Patent Specification 383,195, to reduce the width of the gaps between the disks by means of a pressure-applying member, which is disposed outside the disk-containing space and tends to exhibit a thermal deformation acting axially on the disks. The torque being transmitted by the coupling progressively increases as the width of the gaps decreases.
But the coupling described in that Austrian patent specification involves a high structural expenditure owing to the pressure-applying member, which is laterally disposed, and to the resilient spacers, which are required to equalize the disk spacing. Whereas such a coupling will have the desired torque characteristic during an operation involving liquid friction, this will not be the case during an operation involving metal-to-metal friction, which is desired to occur above a certain temperature and involves a very steep torque rise. That range, which owing to the strong torque rise is called hump range, is important for the protection of the coupling from thermal overloading. But that protection will not be afforded unless the hump torque is sufficiently high. When a vehicle is about to overcome an obstacle off the road and one of the driven wheels is slipping, the hump torque transmitted by a liquid friction coupling connected between the slipping wheel and a non-slipping wheel shall be so high that either the obstacle is overcome or the motor is stalled.
The steep torque rise during an operation involving metal-to-metal friction may be due to the fact, inter alia, that the viscous liquid between two disks is displaced and that the unilateral action of the internal pressure, which increases as a result of the temperature rise, causes the disks of each pair to contact each other. To ensure that state will be assumed, the viscous liquid between the disks must be displaced--for that purpose it is usual to provide the disks with scraping edges--and the unilateral pressure must be sufficient to force the disks of each pair against each other. To produce a hump torque which is as high as possible, the area in which the metallic surfaces actually contact each other must be as large as possible. This means that the disks of each pair must contact each other throughout their confronting surfaces to the largest extent possible.
This is not the case in the coupling disclosed in said Austrian patent specification in spite of the provision of resilient spacers. Whereas the latter equalize the spacing of the inner disks, the positions of the displaceable outer disks relative to the inner disks will remain indefinite and will be at random so that the metal-to-metal friction between the disks of each pair will not be initiated at the same time and will not occur between the disks of all pairs.
The indefinite position of the outer disks (or the indefinite position of displaceable inner disks) will give rise to a wear of the disks also during an operation involving liquid friction and that wear will change the torque characteristics of the coupling in the course of time.
It is known from German Patent Specification 38 28 421 that the scraping action for accelerating the displacement of the viscous liquid between two disks can be improved in that the conventional known scraping edges are omitted and sectors of the disks are bent about radial bend lines. Whereas this will increase the hump torque, the behaviour during an operation involving liquid friction will not be improved. Besides, a larger number of disks will be required and the wear of the disks will be increased.